1. Field of the Invention
The present invention relates to an apparatus and method for reporting channel status in a cellular radio communication system and, in particular, a method and system for reporting the channel status information in a feedback order determined according to time-varying characteristic of feedback information when the reference signal for downlink channel estimation is transmitted at long period.
2. Description of the Related Art
The mobile communication system has evolved into a high-speed, high-quality wireless packet data communication system to provide data and multimedia services beyond the early voice-oriented services. Recently, various mobile communication standards, such as High Speed Downlink Packet Access (HSDPA), High Speed Uplink Packet Access (HSUPA), High Rate Packet Data (HRPD) defined in 3rd Generation Partnership Project-2 (3GPP2), and 802.16 defined in IEEE, have been developed to support the high-speed, high-quality wireless packet data communication services.
The existing 3rd generation wireless packet data communication system, such as HSDPA, HSUPA and HRPD, uses technologies such as Adaptive Modulation and Coding (AMC) and Channel-Sensitive Scheduling to improve the transmission efficiency. With the use of AMC, a transmitter can adjust the amount of transmission data according to the channel state. That is, when the channel state is not ‘Good’, the transmitter reduces the amount of transmission data to adjust the reception error probability to a desired level, and when the channel state is ‘Good’, the transmitter increases the amount of transmission data to adjust the reception error probability to the desired level, thereby efficiently transmitting a large volume of information. With the use of the Channel-Sensitive Scheduling-based resource management method, the transmitter selectively services the user having a better channel state among several users, thus increasing the system capacity compared to the method of allocating a channel to one user and servicing the user with the allocated channel. Such capacity increase is so called ‘multi-user diversity gain’. For example, the AMC technique and the Channel-Sensitive Scheduling method each are a method of applying an appropriate modulation and coding scheme at the most-efficient time determined depending on the partial channel state information fed back from a receiver.
There have been many researches done to adopt Orthogonal Frequency Division Multiple Access (OFDMA) to next generation communication systems in place of Code Division Multiple Access (CDMA) that has been used in 2nd and 3rd Generation mobile communication systems. The standardization organizations such as 3GPP, 3GPP2, and IEEE are developing standards for enhanced system based on the OFDMA or modified OFDMA. It is known that OFDMA promises to increase system capacity as compared to CDMA. One of the factors affecting the increase of system capacity in an OFDMA system is the use of frequency domain scheduling. As the channel sensitive scheduling technique uses the time-varying channel for capacity gain, it is possible to increase the capacity gain with frequency-varying channel characteristic.
FIG. 1 is a diagram illustrating a subframe structure in downlink of the convention LTE system.
Referring to FIG. 1, a subframe 115 is composed of 14 OFDM symbols 100˜113 of which first three OFDM symbols 100˜102 are the region allocated for Physical Downlink Control Channel (PDCCH) and the rest OFDM symbols 103˜113 are the region allocated for Physical Downlink Shared Channel (PDSCH). The PDCCH is transmitted in the PDCCH region 100˜102 across entire system band while the PDSCH is transmitted on Resource Block (RB) 114 as a basic unit of scheduling. Here, each RB is composed of 12 subcarriers, and the total number of the RBs varies according to the system bandwidth. The reason why the PDCCH region is arranged at the beginning of the subframe is for the UE to check the PDCCH first and, if no data is addressed to the UE, enters a micro sleep mode to save the UE's power consumption.
FIG. 2 is a diagram illustrating transmission format of downlink common reference signal (CRS) for 4 transmit antennas in the convention LTE system.
Referring to FIG. 2, a Resource Element (RE) means a subcarrier 203 in one OFDM symbol 201, and an RB 202 is composed of 12×14 REs. The receiver receives CRSs 210˜213 corresponding to the respective antennas 204˜207 to estimate the channel states of the respective transmit antennas 204˜207. For this purpose, the each antenna transmits null signal 208 at RE region on which the other antennas transmit CRSs. That is, the antenna port 0 204 transmits null signal 208 at the RE positions of the CRS 211˜213 transmitted by other antenna ports 1-3 205˜207. In one RB 202, the CRSs 210 and 211 for the antenna port 0 204 and antenna port 1 205 are transmitted on 8 REs respectively while the CRSs 212 and 213 for the antenna port 2 206 and the antenna port 3 207 are transmitted on 4 REs respectively. Accordingly, in 4-antenna case, the CRSs are transmitted using 24 REs per RB 202. The receives estimates channel using the CRS and uses the PDSCH 209 in demodulation or generating downlink channel state information. Here, if the usage of the CRS is restricted to downlink channel state information other than channel estimation for demodulation, it may possible to reduce the number of REs for CRS transmission in one RB 202. This is because the channel estimation error in demodulation is less than the error required in generating downlink channel state information.
The receiver receives the CRS as depicted in FIG. 2 in all of the RBs in every subframe and generates a Rank Indicator (RI), a Channel Quality Indicator (CQI), and a Precoding Matrix Indicator (PMI) representing the downlink channel state.
FIG. 3 is a diagram illustrating uplink periodic feedback format in the conventional LTE system.
Referring to FIG. 3, the UE sends the eNB the RI, CQI, and PMI indicating channel status. In FIG. 3, the parameters P(=Np), K, M, and O determining the feedback format are transmitted to the UE through higher layer signaling and defined and ranged, respectively, as follows.                K: number of representative sub-bands for CQI report, K{1, 2, 3, 4},        O: offset at RI report timing and CQI/PMI report timing, {0, −1, . . . , −(P−1), −P}        M=MRI: RI report period, M {1, 2, 4, 8, 16, 32, OFF},        Np, NOFFSET,CQI: period and offset for sub-band CQI feedback, determined by cqi-pmi-Configuration Index        MRI, NOFFSET,CQI: period and offset for sub-banded CQI feedback determined by cqi-pmi-Configuration Index.        
Here, the sub-band means a group of one or more consecutive RBs. The time-sensitive sub-band CQI feedback period 302 is shorter than the periods of the RI 303 and wideband CQI/PMI feedback 305. Since the sub-band CQI feedback has to be performed for all of the K representative sub-bands in the J frequency band regions, the CQI feedbacks for entire J*K sub-bands are performed when the wideband CQI/PMI feedback occurs at a time as denoted by reference number 304 such that the wideband CQI/PMI feedback period becomes (J*K+1)*P 300 and 301. Since it is not sensitive to time, the RI is fed back at longest period with offset O 306 to avoid transmission collision with other feedback informations.
FIG. 4 is a diagram illustrating downlink reference signal transmission and uplink feedback format in the LTE-Advanced (LTE-A) system.
Referring to FIG. 4, the Channel State Information Reference Signal (CSI-RS) 402 and 403 for downlink channel state estimation are transmitted at frame period 404. That is, the CSI-RS is transmitted at one of 10 subframes constituting the frame 400 or 401. The reason why the CSI-RS is transmitted in this way is because it is used only for the purpose of downlink channel state estimation as aforementioned. Upon receipt of the CSI-RS 402 and 403, the UEs 407˜409 perform feedback of the corresponding channel status informations in uplink after elapse of the minimum calculated time 410 and 411 required for generating the downlink channel status informations as denoted by reference number 412˜414. At this time, due to the long CSI-RS transmission period 404, the receiver feeds back several sub-band channel status informations and wideband channel status information in the transmission period, and the eNB determines the offset of the downlink channel status used in the eNB according to when the UE feeds back which information. Accordingly, it is necessary to design the uplink feedback format in consideration of the characteristics of the channel status information fed back. Also, in case that multiple UEs perform feedback immediately after channel estimation with CSI-RS to reduce the offset of the channel status, uplink overhead may be concentrated in a subframe. There is therefore a need of a design of a feedback format for distributing uplink feedback overhead while maintaining the offsets of the channel status informations fed back by multiple UEs evenly.